Multilayer porous membranes from polymers that can be dissolved in a solvent at room temperature have been produced by casting processes. Typically, such membranes are asymmetric in that one layer has an average pore size different from the average pore size in a second layer. Such membranes are useful when utilizing the layer having a larger average pore size as a prefilter so that larger, retained particles are not concentrated in the layer having the smaller average pore size. When the multilayer porous membrane is utilized in this manner, the overall useful life of the membrane is extended.
Multilayer porous membranes can be formed by co-casting two or more polymer solutions at or near ambient conditions onto a support to form a multilayer liquid sheet which is then immersed in a liquid coagulation bath to effect phase separation and to form a multilayer porous membrane. Since these processes require the formation of a liquid layer, the processes are limited to polymers that can be dissolved in a solvent at normal room temperatures. These casting techniques are common in the art.
U.S. Pat. No. 4,828,772 discloses microporous membranes formed from ultrahigh molecular weight polyethylene (UHMW-PE). UHMW-PE does not dissolve in solvent at room temperature. In this process, UHMW-PE is mixed with a porogen (solvent) which is then heated to form a composition that can be caused to flow under pressure, such as the pressure exerted in a conventional extruder. The porogen has a higher boiling point than the temperature to which the UHMW-PE-porogen mixture is heated to form a flowable composition. A single layer porous membrane is produced from the heated UHMW-PE-porogen mixture by forming it (via extrusion) and cooling it. The porogen is removed from the formed UHMW-PE extrusion with a solvent that selectively dissolves the porogen, while leaving the UHMW-PE unaffected. Heretofore, this technique has only been contemplated for forming single layer porous membranes, for use alone, or in combination with one or more separate porous membranes in a composite filter.
One method for forming multilayer porous membranes is to form each layer separately and join the layers in a filtration device. This method is undesirable, in many applications, since the layers are fragile and can easily deform or fracture during handling to effect joining of the layers. It does, however, have the advantage that each layer may have a different composition from the others.
Accordingly, it would be desirable to provide a process for forming an integral multilayer porous membrane produced from an extrudable polymer, such as UHMW-PE, which does not afford itself to casting techniques. Such a process would permit the use of polymers not currently available for producing integral multilayer porous membranes.